1. Field of the Invention
The present invention relates to a positive photoresist composition suitable for use in lithographies using far ultraviolet rays (including excimer lasers and the like), etc.
2. Related Art
Recently, with a rise in the integration level of integrated circuits, formation of pattern of sub-micron order is required. Particularly, excimer laser lithography is watched with interest because it enables production of 64 and 256 megabit Dynamic Random Access Memories (MDRAMS). As a result of such an alteration of light source from the prior mercury lamp rays such as g ray and i ray to the laser having a wavelength of, for example, 248 nm, the following properties are demanded of today's resists in addition to the hitherto required properties such as heat resistance, film thickness retention, profile, etc.:
(1) a high sensitivity to the short wavelength light sources mentioned above, and PA1 (2) a high resolution.
In view of the above-mentioned state of art, the so-called chemical amplification type resist utilizing an acid catalyst and a chemical amplification effect has been proposed. In this type of resist, an acid is generated from a photo-induced acid precursor upon irradiation with a light, and a reaction takes place due to the catalytic action of the acid and thereby a difference arises between the solubilities of the exposed part and unexposed part in developing solution. As a result, a positive (or negative) photoresist is obtained.
As the alkali-soluble resin in such chemical amplification type resists, poly-(p-hydroxystyrene-p-t-butoxycarbonyloxystyrene) has hitherto been used as mentioned in, for example, Japanese Patent Application KOKAI No. 2-209977. However, this type of resin cannot satisfy the conditions necessary for production of 64 and 256 MDRAM for the following reasons:
(1) such a resin is low in adhesive strength to the silicone substrate, and therefore the positive pattern formed upon development is readily fissile; PA0 (2) in such a resin, a large quantity of carbonium ion is formed at the time when the t-butoxycarbonyl group is eliminated by the action of the acid generated upon irradiation with light, which promotes an inversion from positive type to negative type; and PA0 (3) such a resin cannot give a positive pattern of high resolution.